Thermomechanical processing of mechanically alloyed materials

ABSTRACT

Flat rolled products are produced from mechanically alloyed composite particles by compaction and hot working using prescribed heat treatments, strain, strain rates, etc.

United States Patent 1191 Bomford et al.

THERMOMECIIANICAL PROCESSING OF MECHANICALLY ALLOYED MATERIALS Inventors: Michael James Bomiord, Bublikon,

Switzerland; Robert Lacock Cairns, Suffern, NY.

Appl. N0.: 396,203

[ 51 Oct. 29, 1974 [56] References Cited UNITED STATES PATENTS 3,746,581 7/1973 Cairns et al. 148/115 F [5 7 ABSTRACT Flat rolled products are produced from mechanically alloyed composite particles by compaction and hot working using prescribed heat treatments, strain, strain rates, etc.

8 Claims, 2 Drawing Figures M06? POM/N6 us. Cl l48/11.5 r 1m. c1 B221 3/18 Field of Search 148/1 1.5 F; 75/05 BB, 75/05 BC 95 e Mi I700 /aoo I900 ,POZU/YG Emamvmg 7 THERMOMECHANICAL PROCESSING OF MECHANICALLY ALLOYED MATERIALS The subject invention relates to powder metallurgy, and is particularly addressed to the fabrication of dispersion strengthened mechanically alloyed" alloys, notably the superalloys.

The recently introduced concept of mechanical alloying," described in US. Pat. Nos. 3,591,362 and 3,723,092, incorporated herein by reference, involves, as is known, the dry and intensive milling of powders in high energy machines and under special conditions during which constituent powders are repeatedly fragmented and cold bonded by the continuous impacting action of attriting elements. This processing is continued for a period such that composite product powder particles of saturation, or at least substantial saturation, hardness are formed, the composition of which correspond to the percentages of the respective components in the original charge. By reason of this, the constituent powders become most intimately interdispersed at close interparticle spacings, the composite particles being exceptionally dense and homogeneous, and characterized by cohesive internal structures. A further specific attribute of this unique development is that it permits of precipitation hardening, dispersion strengthening and matrix stiffening to be brought together in one alloy, particularly the nickel and nickel-chromium superalloys.

Virtually all of the thermomechanical processing of mechanically alloyed composite particles has been accomplished by recourse to extrusion, a seemingly indispensible and essential step. There are limitations attendant extrusion, however, and it would be of benefit to have at ones command such techniques as hot rolling to produce desired mill products, among which might be mentioned plate, strip, and sheet. In retrospect, it has probably been the anticipated difficulties expected in connection with hot rolling which may have rather self-mandated or at least encouraged turning to extrusion. This would not have been misplaced for we have found that a considerable number of parameters are involved in ultimately achieving an acceptable flat mill product such as those above-mentioned, including (i) the nature of the mechanically alloy composite particles (ii) compaction temperature, (iii) rolling temperature, (iv) percentage reduction (strain), (v) strain rate, (vi) product thickness, etc.

In any case, we have discovered that satisfactory flat products, including plate, strip, sheet and the like, can be produced from mechanically alloyed materials, provided that the processing parameters described herein are observed. Superalloy sheet having good stress rupture properties at elevated temperatures (l900F.) has been produced, the sheet being of a thickness of but 0.02 inch. Moreover, these properties obtain in both the longitudinal and transverse direction. Needless to say, such characteristics greatly contribute to the production of gas turbine combustion, turbine and after burner components, applications requiring an alloy to be in sheet form and capable of withstanding the demands imposed by stress at high temperature.

Generally speaking, the present invention contemplates subjecting mechanically alloyed" dispersion strengthened composite alloy particles to a processing sequence involving (i) compacting the composite particles at a temperature above l,700 but less than 2,l0()F., and most advantageously, over the range of about l,850 to 2,000F. and thereafter (ii) hot working as by rolling the compacted billet so produced at a temperature above the recrystallization temperature of the alloy and up to 2,050F., with a temperature range of about 1,800 to 2,000F. being deemed beneficial, the rolling operation being conducted at (iii) a strain rate of from about 3 to 4 up to 30 or 40 in/in/sec and with (iv) the total (both longitudinal and transverse) percentage reduction in thickness of the compact being at least percent, preferably at least percent, and up to 97 percent. To develop stress-rupture properties, the alloys are thereafter (v) heated at a temperature causative of germinative grain growth (secondary recrystallization) such that the alloys take on a structure comprised of course elongated grains and which is characterized by an absence of fine grains.

MECHANlCALLY ALLOYED PARTlC LES Prior to the compaction step, milling of the powder undergoing mechanically alloying should be continued until cold worked composite metal powders are produced characterized by markedly increased hardness (that is, the powder contains a substantial amount of stored energy).

COMPACTION Compacting of the mechanically alloyed particles can be accomplished by known means, including hot pressing, extruding using a blank die, press forging of loose powder in a box, etc. The point of concern is that an excessively high temperature, i.e., 2,100F. and above, be avoided. High compaction temperatures tend to anneal out an undue amount of the composite particle energy imparted by way of the mechanical alloying phenomenon itself. This lends ultimately to products having an unnecessarily high amount of fine grains which in turn detract from stress-rupture strength. On the other hand, compaction temperatures below l,700F. can result in failure to achieve full densification or cold cracking in the compacted billet. While a temperature range of l,750 to 2,050F. may be employed it is much preferred to use a range of l800 or l850 to 2000F.

HOT WORKING a. Temperature The working operation is preferably conducted by hot rolling. As is the case with compacting, should the rolling temperature be to the excess, i.e., 2050F. and above, too much energy will be annealed out due to dynamic recovery during rolling. The rolling temperature should not exceed 2,050F. and it is to advantage that it be maintained below about 1,950F. This excessive recovery can also ensue from a combination of relatively high rolling temperature, say 2,000F. and above plus a relatively high strain rate, e.g., about 15 or 20 in- /in/sec or more, which can induce further adiabatic heating in the material.

Should the rolling temperature be needlessly too low, then overworking, especially at higher working strain rates, too easily can be the result with a concomitant drop in properties. Too, the alloys tend to manifest a propensity toward or become more susceptible to cracking. A rolling temperature of 1,825 to l,975F. is deemed most satisfactory. It might be mentioned that alloys in the overworked state tend to recrystallize in an equiaxed manner or to a fine grain size. On the other hand, a final state of underworking leads to an alloy which virtually will not undergo secondary recrystallization.

b. Strain rate As above indicated, a strain rate of about 3 to 4 and up toabout 30 or 40 in/in/sec is considered satisfactory. With strain rates below about 3 in/in/sec, it is considered that the imparted strain energy is somewhat balanced by dynamic recovery so that germanative grain growth will not take place. Strain rates much above about 40 in/in/sec tend to cause overworking at the lower working temperatures, or excessive adiabatic heating and recovery at the higher working temperatures. A strain rate of about l5 to about 30 in/in/sec is considered quite suitable.

c. Strain The total reduction in thickness (strain) required is at least 75 percent. It has been found that in the absence of sufficient strain, recrystallization response upon the application of germinative grain growth treatment is impaired. Coarse grains were obtained but they were not elongated. To the other extreme, a total reduction in thickness much beyond 97 percent is an invitation to overrolling and attendant consequences. A total strain of about 80 to 91 percent reduction in thickness, (Le, a log rolling strain of about 0.5 to 1.2) is decidedly of particular benefit.

d. Rolling temperature, strain and rolling direction In striving for optimum results, it is to be emphasized that rolling temperature, strain and direction of rolling are not independent. Rather, they are interrelated. This is reflected by the curves in FIGS. 1 and 2, the former being directed to unidirectional rolling with the latter involving cross rolling. It can be seen that a rolling temperature of 1,850F. and a strain of LI (approx. 90.5 percent reduction in thickness) is not recommended for unidirectional processing due to overrolling, although this combination would be generally acceptable for cross rolling. While the rolling temperature would be satisfactory in both instances, the strain would be unnecessarily high for unidirectional rolling. Accordingly, rolling temperature and strain should be related to strain so as to represent a point falling within the curves AB and CD of FIG. 1 and curves EF and GH of FIG. 2, depending upon rolling direction.

e. Product Thickness The products contemplated herein must be heat treated to develop the coarse elongated grain structure necessary for elevated temperature use. However, when the final thickness of the flat product to be produced is less than about 0.1 inch thick, then recrystallization should be induced prior to completion of the hot working. The reason for this is to prevent formation of elongated grains which would extend the full thickness of the material. Through thickness grain boundaries, so formed. are deleterious to strength and ductility of the sheet product. Incident to this, the compact billet should be hot reduced in thickness at least about 75 percent, preferably at least 80 percent, before the grain growth treatment is applied. Rolling is thereafter completed.

f. Heat Treatment.

The germinative grain growth treatment should be conducted at a temperature above about 2,200F. and below the incipient melting temperature, depending upon the given alloy composition. A range of 2,350 to TABLE I Cr Al Ti Zr B Y,0, Ni Alloy '71 a a 01 Bal. balance nickel plus impurities NOTE: nominal composition given The billet-compacts were cog rolled to square and then rolled directly to plate or sheet, or were sectioned and ground to provide two parallel faces to which mild steel plate was welded. The billets and canned blocks were hot rolled to the various reductions and at the different temperatures reported in Tables II through IV. In producing the'thinner sheets several plates stacked in a sealed mild steel container were rolled, (the container can be stainless steel, nickel or similar material). To prevent adhesion, the plates were separated from one another using A1 0 powder (a suspension of glass or ceramic material may be used as the separation compound). The rolled sheets parted rather easily.

The rolled plates and sheets were then heat treated at 2,400F. in an effort to achieve a coarsened, elongated structure and thus optimize stress-rupture properties. This treatment was applied after final rolling where the desired thickness was greater than 0.1 inch;

otherwise, it was applied at an intermediate rolling stage to obviate having the grain boundaries completely transversing the product thickness.

Stress rupture strength and ductility were determined at 1900F. The criterion for an acceptable alloy product was an approximate 1,000 hour stress rupture life or higher at the l,900F. temperature under a stress of 10,000 psi.

EXAMPLE I /a-inch plate (perhaps more accurately sheet) was produced from Alloy l. The rolling schedule and properties are given in Tables II and III, respectively, the billet having been compacted at l,900F., with the starting thickness being 1 /2 inches.

TABLE II Rolling Schedule NOTE: Longitudinal direction parallel to cylinder axis of compacted billets. Total rolling reduction approx. 91%.

TABLE 111 Longitudinal TransveTse m Approx. 1000 Approx. 1000 Rolling Stress Life El. hr. Rupture Stress Life El. hr. Rupture Test Temp. (F.) (ksi) (hours) (ksi) (ksi) (hours) (ksi) o 2Q W59 V B 1900 19 235.8 2.2 18.5 19 145.6 3.5 18.5

Mechanical Alloying Parameters:

Attritor: 100 gallon capacity Speed: 75 rpm Time: 22 hours Unbroken stress raised Regarding Table 111, it will be observed that stress- The stress-rupture strengths obtained in the longiturupture strength was quite high irrespective of rolling dinal direction are rather noticably higher than those direction. The best combination of biaxial strength was obtained in the transverse direction. It is considered obtained in connection with the 1.900F. roll, although that cross rolling during stack rolling would have minithe 1,800F. roll offered a higher transverse strength, mized the disparity. In any event, the highest rupture with the 2,000F. rolling temperature being more apstrengths were obtained at rolling temperatures of propriate for the longitudinal test. Properties were 1,800 and 1,900F. markedly falling off in respect of the 2100F. rolling 25 temperature. EXAMPLE Following the general procedure of Example 11, EXAMPLE sheets about 0.020-0.03 inch thick were formed using This example illustrates production of thin sheet Alloy l with the test data being reported in Table V. (0.05 i ch), Again, quite satisfactory results were obtained.

m 7 TABLE V '7 v v V m Approx.1000

Rolling Stress Life El. hr. Rupture Test Test Temp. (F.) (ksi) (hours) (ksi) Direction 11 1900 15 44.9 0.9 13 Longitudinal 15 4.7 0.9 12 Transverse Mechanical Alloying Parameters:

Miami? 100 gallon capacity Speed: 5 rpm Tim: 22 hours Plate of Va-inch thickness was produced from Alloy '7 EXAMPLE 71V 1 as described in Example 1 above. The plate was heat 7 W treated at 2400F. for one-half-hour and the mild steel Plates f l y 2 W re lso prepared to a thickness of can pickled ff Three such plates were cut into 5" about one-fourth-inch for atotal reduction in thickness squares, stacked and sealed into a mild steel container. of about 90 percent using three different rolling sched- Al O was used as parting compound. The piece was ules as follows: unidirectionally rolled to three-eighths inch thickness Schedule 1 in two passes. At this point the sheets were about 0.05 i. at 1950F., 57 percent reduction in longitudinal di- 0.06 inch thick. In Table N there are given the results rection, plus of tests at 1.900F.. three separate hot rolling temperaii. at 1950F., 40 percent reduction in longitudinal lures being reported. direction, plus TABLE IV Longitudinal A Transverse pprox. 1000 A rox. 1000 Rollmg Stress Life El. hr. Rupture Stress Life El. Rupture Test Temp. F.) (ksl) (hours) 711 (ksi) (ksi) (hours) (ksi) E 1800 15 306.5 1.8 l6 15 92.2 1.8 l4

Mechanical Alloying Parameters:

Attritor: 100 gallon cnpneity Speed; rpm Tune: 22 hours Unbroken stress raised.

As a practical matter, only a very small amount of such dispersoids need be employed, e.g., up to percent by volume.

Finally, it will be understood that modifications and S h d l 1| 5 variations of the invention might be resorted to without i. at 1950F., 42 percent reduction in longitudinal P Pg from e 5Pitit and Scope thereof as those d 29 percent i transverse di i l skilled in the art will readily understand. For example, ii. at l950F., 40 percent reduction in longitudinal ppf p annealing treetments e be p y direction, plus during the overall hot working operation. Such are coniii. at l950F., 47 percent reduction in transverse di- 10 stdeted to be within the Rurvlew and Scope of the rection, plus vention and appended claims.

i v. at 1900F., 25 percent reduction in longitudinal we elalmi direction 1. A process for producing flat mill products from Schedule m mechanically alloyed composite powder particles i. at l800F., 35 percent reduction in longitudinal diwhich eomprtses eotttpaetmg the Composite ParticleS at rection plus a temperature within the range of about l,700F. to

ii. at l800F., 37 percent reduction in transverse di- ZiIOOOF-i hot tenmg the e p y at a p rection, plus ture above the recrystallization temperature of the iii. at 1800F., 40 percent reduction in transverse dialloy but below about. 1 and at a Strain of from motion, plus about 3 to about 40 in/in/sec, and continuing the hot iv. at l800F., 50 percent reduction in longitudinal roihng operauon untll the percentage reduction in direction, plus thickness of the compacted body is at least 75 percent v, at l800F., 23 percent reduction in longitudinal and up to 97 P direction. 2. A process in accordance with claim 1 in which the O 0 The sheet materials were heat treated for one hour at cogllxlctlon m m 135.0 290.0 2,400F. for germinative grain coarsening and 24 hours t ancie l Claim 2 m whlchothe at 1,300F. for aging and thereafter tested for stresseanpzrature o m to 9 F 29.00 rupture behavior at l,900F. Test results are set forth Str z q C mm 1 m Whlch the in Table Vl, together with the rolling schedule and api e a i g i proximate total reduction in thickness. process n accor ance C mm 1 m whlch the TATBT T W "fie Total Reduction From Overall Total Est. Total Estimated l900F., 100 Hr. Test Rolling Ori inal Thickness Reduction in Reduction in Area Rupture Strength (ksi) Schedule Cong. lrans. Thickness Longitudinal Long. Trans.

1 l 90 0 90 85 12.9 l0.9 J ll 73 i7 90 so l0.7 l0.9 K Ill 50 62.5 92 7a l2.5

Mechanical Alloying Parameters:

Attritor: 100 gallon capacity Speed: 75 rpm Time: 22 hours Among the alloys to which the invention is applicable percentage reduction in thickness is from about 80 to are those containing up to 65 percent, e.g., l to 25 or about 91 percent. percent, chromium; up to 30 percent, e.g., 5 to 25 6. A process in accordance with claim 1 in which if percent, cobalt; up to 10 percent, e.g., l to 9 percent, the hot rolling is accomplished unidirectionally, the aluminum, and up to 8 percent, e.g., l to 7 percent, tirolling temperature and strain are correlated such as to tanium, particularly those alloys containing 6 percent represent a point within the curves AB and CD of FIG. or more of aluminum plus titanium; up to 30 percent. 1 of the accompanying drawing and if the hot rolling is e.g., l to 8 percent molybdenum, up to 25 percent, e.g., conducted in two directions, the rolling temperature 2 to 20 percent, tungsten, up to 10 percent columbium, and strain are correlated to give a point within the and up to 10 percent tantalum, particularly those concurves EF and GH of HO. 2 of the drawing. taining at least 4 percent or in total of Mo W Cb 7. A process in accordance with claim 2 in which the Ta; up to 2 or 4 percent zirconium; up to 0.5 percent hot rolling temperature is from 1,800 to 2,()()0F., the e "P to 5 Percent hafnium; P 2 Percent Vatla' strain rate is from about 15 to 30 in/in/sec and the perdlumi "P to 6 Percent P pp "P to 5 P n mange eentage reduction in thickness is about 80 to about 9| nese, up to 70 percent iron; up to 4 percent silicon, and the balance essentially nickel Cobalt base alloys of percent similar composition can be treated. Superalloys and ptiacessm g g 3 i 7 i whlchtthe other contemplated alloys can contain up to, say, 10 pm therea eat tea 6 a empem w percent or more by volume of a refractory dispersoid from about 2 to f the melplent meltmg material including the oxides, carbides, nitrides and botemperature of the alloy to achieve a coarse elongated rides. Such refractory dispersoids can be of various elements including yttrium, lanthanum, thorium, zirconium, hafnium, titanium, silicon, aluminum, cerium,

uranium, magnesium, calcium, beryllium and the like. I

grain structure. 

1. A PROCESS FOR PRODUCING FLAT MILLPRODUCTS FROM MECHANICALLY ALLOYED COMPOSITE POWDER PARTICLES WHICH COMPRISES COMPACTING THE COMPOSITE PARTICLES ATA TEMPERATURE WITHIN THE RANGE OF ABOUT 1,700*F. TO 2,100*F., HOT ROLLING THE COMPACTED BODY AT A TEMPERATURE ABOVE THE RECRYSTALLIZATION TEMPERATURE OF THE ALLOY BUT BELOW ABOUT 2,050*F. AND AT A
 2. A process in accordance with claim 1 in which the compaction temperature is from 1,850* to 2,000*F.
 3. A Process in accordance with claim 1 in which the temperature of hot rolling is from 1,800* to 2,000*F.
 4. A process in accordance with claim 1 in which the strain rate is about 15 to 30 in/in/sec.
 5. A process in accordance with claim 1 in which the percentage reduction in thickness is from about 80 to about 91 percent.
 6. A process in accordance with claim 1 in which if the hot rolling is accomplished unidirectionally, the rolling temperature and strain are correlated such as to represent a point within the curves AB and CD of FIG. 1 of the accompanying drawing and if the hot rolling is conducted in two directions, the rolling temperature and strain are correlated to give a point within the curves EF and GH of FIG. 2 of the drawing.
 7. A process in accordance with claim 2 in which the hot rolling temperature is from 1,800* to 2,000*F., the strain rate is from about 15 to 30 in/in/sec and the percentage reduction in thickness is about 80 to about 91 percent.
 8. A process in accordance with claim 7 in which the product is thereafter heat treated at a temperature from about 2,200*F. to below the incipient melting temperature of the alloy to achieve a coarse elongated grain structure. 